The cascaded arc : high flows of rovibrationally excited H2 and its impact on H- ion formation

The cascaded arc is a plasma source providing high fluxes of excited and reactive species such as ions, radicals and rovibrationally excited mols. The plasma is produced under pressures of some kPa in a d.c. arc with elec. powers up to 10 kW. The plasma leaves the arc channel through a nozzle and expands with supersonic velocity into a vacuum-chamber kept by pumps at low pressures. We investigated the case of a pure hydrogen plasma jet with and without an applied axial magnetic field that confines ions and electrons in the jet. Highly excited mols. and atoms were detected by means of laser-induced fluorescence and optical emission spectroscopy. In case of an applied magnetic field the at. state distribution of hydrogen atoms shows an overpopulation between the electronic states p = 5, 4 and 3. The influence of the highly excited hydrogen mols. on H- ion formation and a possible mechanism involving this neg. ion and producing at. hydrogen in state p = 3 will be discussed